1. Field of the Invention
The present invention relates to a system for determining the position of a test object, and in particular for determining the position of the centers of curvatures of a lens system.
2. Description of Related Art
Known from the prior art are focussing devices and autocollimation telescopes for measuring changes in location and angle and centering errors of lenses and lens systems, in which a beam splitter and an objective lens, which acts as a collimator and telescope, are used to evaluate a suitable projected image following reflection at the surface to be examined. Known evaluation units are, for example, two-dimensional sensor units or, in the classic case, the human eye. If the centre of curvature of the examined surface (sphere) is not exactly on the reference axis, the illumination beams do not impinge perpendicularly upon the surface of the testpiece. As a result, following reflection at the surface of the testpiece, they do not return into themselves, but are reflected at a reflection angle that deviates from 90° to the surface. The deviation is directly proportional to the centering error of the examined surface (sphere).
In this case, in order to describe the position of a lens or cemented element, or a group of elements, it is necessary for two surfaces to be measured. If this is to be related to further surfaces, these further surfaces must also be measured. This object equates to the object of imaging the image of a mark on to any given location along an optical axis. To achieve this object, it is known from the prior art to use exchangeable supplementary optical systems, or to change the imaging distance of the autocollimation telescope by means of an internal focussing system (e.g. by means of lenses or lens groups that are displaceable along the optical axis), or to achieve, by changing the distance (linear guide) between the autocollimation telescope and the surface to be examined (testpiece or test object), that the image from the reflection of the surface of a mark from the autocollimation telescope that is to be examined appears again in the plane of the mark, such that it can be received by a detection unit.